The long-term goal is to characterize neural mechanisms that allow the auditory system to identify "sound sources" that overlap in frequency and time. Normal-hearing human listeners are remarkably good at hearing individual sources, even though they cannot be easily distinguished in the power spectrum of the composite waveform. Some of the difficulty that hearing impaired listeners have in understanding speech in noisy environments may be due to an inability to identify sound sources correctly. Understanding the neural mechanisms that underlie sound source identification in the normal auditory system is essential for developing signal processing strategies that preserve or restore that ability. Measurements of the responses of IC neurons to harmonic and mistuned complex tones will be made. Previous studies have shown that the mistuned tones elicit distinctive temporal discharge patterns from IC neurons. Responses in other brainstem nuclei will also be studied, to determine what temporal information is delivered to the IC, and how that information is modified in the IC. Pharmacological methods will be used to determine how IC neurons integrate that information. All studies will test specific hypotheses derived from a model for spectral integration developed previously. According to this model, the temporal discharge patterns of IC neurons result from the combination of excitatory and inhibitory inputs, each of which is phase-locked to the stimulus envelope in a narrow frequency band. For harmonic tones, all envelopes have the same periodicity, determined by f0. Mistuning changes some but not all input envelopes, and the weighted sum of these inputs with different periodicities gives rise to the distinctive modulated temporal discharge patterns exhibited in the IC. There are three Specific Aims. Specific Aim 1 is to measure the representation in the IC of harmonic complex tones, and the same tones with a "mistuned" component. Specific Aim 2 is to measure the responses of neurons in other brainstem nuclei to the same harmonic and mistuned complex tones, to determine what excitatory and inhibitory information is sent to the IC. In Specific Aim 3, responses of IC neurons will be measured before, during, and after iontophoretic application of GABAa agonists and antagonists, to further test the hypothesis that temporally-patterned inhibition produces the stereotypical responses observed in the IC.